MSVC don't like forwarded classes

[tower120]

The following code does not compiles with MSVC 15.9.3

#include <iostream>
#include <memory>
#include <vector>

#include <range/v3/view/indirect.hpp>


class Data;

struct Test {
    std::vector<Data*> m_list;

    auto list() {
        return m_list | ranges::view::indirect;
    }
};


class Data {};

int main()
{
    std::cout << Test().list().size();
}

With C2139 error, saying that undefined Data class can not be used with "__is_convertible_to".
LLVM toolchain do compiles this code.

[CaseyCarter]

Ouch. The implementation of common_type evaluates std::is_convertible<Data, const Data&> which violates the (defective) precondition of is_convertible (http://eel.is/c++draft/meta.rel#2.sentence-13) that the argument types are "complete types, arrays of unknown bound, or cv void types." And this despite that the specification for is_convertible in [meta.rel]/5:

The predicate condition for a template specialization is_­convertible<From, To> shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

To test() {
  return declval<From>();
}

is perfectly well-formed when To is const T& and From is T for an incomplete type T since declval<From>() is an xvalue with type T.

I hate bug reports that mean I have to fix the C++ Standard, file a compiler bug, and make a workaround for range-v3. But thank you ;)

[CaseyCarter]

• LWG issue filed here.
• MSVC bug - really an enhancement request to implement the proposed resolution of the LWG issue - has been fixed in the current development compiler. The fix will be in VS 2019. (Maybe Preview 3? I've lost track of the preview release schedule.)
• vcpkg PR submitted